For example, JP-A-2003-298751 discloses a technique to realize a group call among three or more persons in an existing telephone system without a large technical change of a switch and/or a repeater, which constitute the telephone system. That is, the technique includes a step of registering a caller information including a user ID, a telephone number and a password of the caller, and telephone numbers of a plurality of calling destination receivers for which the caller desires to call into a telephone directory database connected to a communication network in advance; a step of selecting the calling destination receiver of the caller, which is registered in the telephone directory database, by accessing to the telephone directory database by the caller; a step of transmitting, by the telephone directory database, the telephone number of the caller and the telephone number of the calling destination receiver selected by the caller to a call center having simultaneous line connection means; and a step of simultaneously calling, by the call center, to the telephone number of the caller and the telephone number of the calling destination receiver selected by the caller through the simultaneous line connection means, and it is possible for the caller to simultaneously call with the plurality of selected calling destination receivers. However, an idea to change a communication mode of the conference on the way is not disclosed.
In addition, U.S. Pat. No. 7,233,589 discloses a technique, which applies the instance messaging (IM) technique to a teleconference. Specifically, presence information of each IM client, usable media and user information are managed by an IM server, and each IM client can obtain such information. When carrying out a text chat, the IM server manages the connection between each participating IM client and the IM server, and merges text data from each participating IM client to deliver the merged result to each participating IM client. When carrying out a voice chat, an AP server manages the connection between each participating IM client and an MD server, and the MD server mixes the voice from each participating IM client except a target IM client to deliver the mixed result to the target IM client. This processing is carried out for each participating IM client. However, this publication only indicates a typical usage method of the presence technique (a usage method of indicating states of clients such as off-line or during IM), and there is no special usage method of the presence data.
Recently, a technique (PoC system) of carrying out a teleconference by using portable terminals such as cellular phones comes to be proposed. During such a teleconference, it may be desirable that the teleconference is carried out while referring to image data with the participants. Moreover, it may be desirable that the reference authority to the image data is limited to some members of the conference participants.